The primary attributes of holographic digital data storage systems are the large data storage density and capacity, fast access times and extremely fast data transfer rates. During the recording process, a beam from a laser is split into two beams: a signal beam, which is used to encode data, and a reference beam. The signal beam passes through an electronic mask, called a spatial-light modulator (SLM), that represents data with a matrix of squares; each square can be directed to transmit light, corresponding to a binary 1, or to block light, corresponding to a binary 0. The signal beam is then focused onto the holographic medium, typically a crystal, where it intersects with the reference beam to create an interference pattern. This interference pattern interacts with the holographic medium in such a way that the original reference beam is incident in the original signal beam, which is then refocused onto a detector array similar to that used in electronic cameras.
The nature of holography is such that the reference beam must be incident at the same angle used to record the data for efficient output. This puts stringent requirements on the optical alignment, but can be used to derive an advantage by allowing more than one hologram to be recorded in the same volume by using reference beams at different angles. The total capacity of such a system is the capacity per page multiplied by the number of individual pages stored in the same volume. Capacity can be further increased by using multiple volumes of media.
For information on conventional volume holographic storage see for example U.S. Pat. No. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages are stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 10.sup.6 bits within one page can be stored and retrieved simultaneously.
High capacity is possible because of the volume nature of holography; other technologies use single or multiple surface access. High transfer rates are possible because of the parallel manner in which data are recorded and replayed; arrays of up to 1K by 1K are available, and will continue to increase in size. Fast access is possible because the angle of the reference beam can be changed electronically or optically, rather than by mechanical motion.
An example of a conventional holographic storage and retrieval system is shown in FIG. 1. An illumination beam from a laser 102 is divided into a signal beam 108 and a reference beam 106 by a beam splitter 104. Both the signal beam 108 and the reference beam 106 are expanded by telescopes 112 and 113 respectively. The signal beam 108 passes through a transmissive SLM 114 to form an encoded signal beam 116. A lens 118 and a holographic storage medium 120 are positioned such that the Fourier Transform of a page of SLM 114 is in the center of the holographic storage medium 120. During recording, an expanded reference beam 117 interacts with the encoded signal beam 116 inside the holographic storage medium 120 to form a hologram. During readout, the reference beam 117 is incident on the holographic storage medium by itself. The diffraction of the reference beam 117 by the hologram forms a reconstructed signal beam 119, which passes through a lens 122 and is incident on a CCD detector array 124. The Fourier transform of the SLM 114's data is inversely Fourier transformed by the lens 122, and the reconstructed image of the SLM 114's data is recorded by the CCD detector array. Notice that the encoding device SLM 114, Fourier transform lens 118, and the inverse Fourier transform lens 122 are on the optical axis, which is characteristic for conventional refractive imaging systems.
From physical considerations of the holographic process, it is desirable to pack as many bits as possible into a data page. This implies imaging a high resolution SLM with, for example, 1024.times.1024 pixels onto the CCD array, which is a formidable optical design problem, and difficult to solve with a lens system. In the past, optical designs have incorporated standard Fourier Transform lenses, which are costly, bulky and are generally optimized for on-axis performance. It would be highly desirable from a commercial and technical perspective to utilize a simple optical system that has good off-axis performance, is cheap and does not require very strict alignment tolerances. This would allow large SLM devices to be imaged onto large CCD arrays, having small pixel sizes. Alternatively, it would allow us to use a number of SLMs to be simultaneously used to synthesize extremely large page sizes not easily achievable with single large arrays. Such a system would have extremely fast transfer rates.
Conventional multi-element refractive imaging systems are ineffective for parallel channel architectures. Additional channels must be sent off axis, which causes severe aberrations, seriously limiting either the page size or the number of interconnects. While a number of parallel interconnect geometries can be designed assuming perfect optics, none have been demonstrated because of the difficulty of imaging multiple megabit pages through a traditional, multi-element lens.
An imaging system using all reflective optics has been disclosed by G. A. Reed in the U.S. Pat. No. 3,190,171. The prior art teaches the construction of a viewing device using a relay imaging system. This relay imaging system uses concave and convex reflectors. Similar systems have also been taught in U.S. Pat. Nos. 4,796,984, and 4,293,186. The concave-convex-reflector imaging system has excellent off-axis optical performance. Application of this system to lithography technology has been taught in U.S. Pat. No. 3,748,015, and in A. Offner's article: "New Concepts in Projection Mask Aligners", OPTICAL ENGINEERING, Vol. 14, No. 2, 1975.